U.S. Provisional Application No. 60/813,741 filed Jun. 14, 2006 is incorporated herein in its entirety by reference.
A properly functioning air conditioner is vital for the cab on a tractor loader backhoe. Between the large area of window glass, and the high heat dissipation of the hydraulics and power train, a backhoe air conditioner is required to perform adequately on a regular basis.
The standard air conditioning system relies on five components to create the cool atmosphere: compressor, condenser, thermal expansion valve, evaporator, and receiver drier. Protection of these components from excessive overpressure on the high side, or vacuum on the lower side, is vital. This is normally accomplished by the use of high and low pressure switches. These switches are placed in locations to provide the necessary system safeguards. The high pressure switch on a tractor loader backhoe is generally located somewhere between the outlet of the compressor and the inlet of the receiver drier, and the low pressure switch is generally located at the outlet of the evaporator. The high pressure switch ensures that the compressor will not over pressurize the system, and cause premature failures of the compressor, condenser, and/or refrigerant lines. The low pressure switch ensures that the compressor will not operate with a vacuum inlet condition, which will cause premature failure of the compressor, as well as provide an opportunity to draw outside contamination into the system.
Monitoring and the controlling of the air conditioning system can be set up many ways. Most commonly, and the way that was initially in use on the tractor loader backhoe is shown in FIG. 3. This method energizes a normally closed relay with the output directly sent to the compressor. Voltage supplied to the relay, is dependent on the position of the air conditioning thermostat, and the blower switch. In addition, the voltage supplied to the relay is sent in parallel to both the high and low pressure switches. The high and low pressure switches of the system are set to be normally open during normal operating conditions. This maintains pressures less than 400 psi on the high side, and pressures greater than 4 psi on the low side.
When an abnormal operating condition occurs, (pressure drops above or below the switches' operating threshold) either switch will actuate and send a voltage to the electromagnet of the relay, causing the circuit to open. When this occurs, the armature of the relay is latched in the open position, and the compressor will remain off, until the system is manually cycled off and back on.
The cab of the loader backhoe sustains a quite high cab heat load, due to the large area of window glass, and the high heat dissipation of the hydraulics and power train. Therefore when there are outside ambient temperatures below 50° F., but the sun is shining bright, there will be cases where the air conditioning needs to be used. The thermodynamics of the air conditioning system is such that the cool ambient temperatures will create a greater pressure drop than usual, causing even more reliance on the low pressure switch. In addition to these cool weather conditions, constant internal cab temperature fluctuations caused by operators opening and closing windows/doors, and turning the heat or A/C on and off, will also create a low side pressure drop in the system that may actuate the low pressure switch.
With the previous system, this can lead to customer dissatisfaction. Every time there is a slight low side pressure drop, the air conditioning relay will latch out, causing the air conditioning system to shut down, and the operator is required to divert his or her attention, and physically turn the A/C system off and back on. These low pressure latch outs are brought to the operator's attention by a warning light on the side console. This is the same warning light that also warms the operator of a high side pressure rise.
Thus, what is sought is a low-pressure cut-out circuit for an air conditioner, which protects the air conditioning system components when subjected to the above low pressure conditions, yet which overcomes shortcomings of the know systems and provides advantages thereover, including avoiding the necessity for manually resetting the system after low pressure cut-out, and providing an ability to better diagnose problems with the system, so as to be more satisfactory when subjected to operating conditions such as described above.